The present invention relates generally to galvanized steel strip, and more particularly to ultrahigh strength galvanized steel strip and to a method for producing such a strip.
Normally, the term "ultra-high strength" refers to steel having a tensile strength greater than 100,000 psi (689 MPa). However, as used herein, the term "ultra-high strength" refers to steel having a tensile strength substantially greater than 120,000 psi (827 MPa).
There are many fabricated steel products, which, in use, require ultra-high strength. Many of these same fabricated steel products are exposed to corrosive conditions and therefore require a relatively high resistance to corrosion. Examples of fabricated steel products which require a combination of corrosion resistance and ultra-high strength include automobile bumper reinforcement beams and automobile door intrusion beams. Such fabricated steel products are usually formed from steel strip.
High strength can be imparted to a steel strip by incorporating certain alloying elements into the strip, but that increases the cost of the steel. Ultra-high strength plain carbon steel, without costly strength-imparting alloying additions, is available in strip form; it is known as martensitic, plain carbon steel strip, and it is disclosed in McFarland U.S. Pat. No. 3,378,360, which describes both the composition of the steel and the method for producing the steel strip so as to impart ultra-high strength thereto. The disclosure of the aforementioned McFarland patent is incorporated herein by reference. Ultra-high strength plain carbon steel, without alloying additions, is less expensive than ultra-high strength steel containing alloying additions for imparting ultra-high strength thereto.
Corrosion resistance can be imparted to steel strip by galvanizing, i.e. coating the steel strip with zinc or a zinc alloy. One conventional process for galvanizing steel strip is the hot dip process in which the steel strip is dipped in a bath of molten coating metal. The hot dip process is continuous and requires, as a preliminary processing step, pre-treating the steel strip before the strip is coated with zinc or zinc alloy. This improves the adherence of the coating to the steel strip. The pre-treating step can be either a preliminary heating operation or a conditioning operation in which the strip surface is conditioned with an inorganic flux. When a fluxing operation is employed, the preliminary heating operation is eliminated. When martensitic, plain carbon steel strip is subjected to heating, either in a conventional preliminary heating operation, or in the course of coating the strip with zinc or zinc alloy in a molten coating bath, or both, the steel strip loses much of its ultra-high strength.
There is another procedure for galvanizing steel, known as electro-galvanizing, which deposits a zinc coating on the steel strip, or on the fabricated part, by electroplating, rather than by hot dipping. When the steel undergoes electro-galvanizing, it is neither subjected to a preliminary heating or fluxing operation nor dipped into molten zinc or zinc alloy; therefore the steel strip loses substantially none of its strength during the coating operation. It is not commercially practical to electro-galvanize martensitic, plain carbon steel strip in a continuous operation because of a slight, inherent waviness in martensitic, plain carbon steel strip which precludes one from running that strip through a conventional high speed electro-galvanizing line. Instead, electro-galvanizing would have to be performed on the product fabricated from the martensitic plain carbon steel strip. However, electro-galvanizing is a relatively expensive operation, and at zinc coating weights above about 30 g/m.sup.2 /side, electro-galvanizing of steel products of the type under consideration here is not commercially practical.
A coating weight of 30 g/m.sup.2 /side or less is not sufficiently heavy to impart the desired corrosion resistance to products of the type under consideration here, which usually require a zinc coating weight substantially greater than 30 g/m.sup.2 /side, and preferably greater than 60 g/m.sup.2 /side. Moreover, products of the type under consideration here are subjected to deforming operations, during their fabrication, which result in at least one inside corner being formed on the product, and inside corners are difficult to coat without coating deficiencies when the coating is applied by electro-galvanizing. Thus, electro-galvanizing of such a product has at least two drawbacks: a relatively light coating weight throughout and a deficient coating at inside corners. The net result is that products subjected to electro-galvanizing have less corrosion resistance than if the same product were subjected to galvanizing by hot dip coating.
One cannot eliminate the preliminary pre-treating step during the conventional continuous hot dip galvanizing process, because this step is necessary in order to assure an adherent, galvanized coating on the steel strip.